Treatment of aromatics containing hydrocarbon feed streams to selectively remove the aromatics therefrom is a conventional hydrocarbon treatment process. The aromatics are typically removed by the use of solvents which are selective for the aromatic hydrocarbons. Solvents which have been used for that purpose have included phenol, furfural, sulfolane, and n-methyl pyrrolidone to name just a few of the better known or more widely used solvents.
The treatment process usually involves the countercurrent contacting of the aromatics containing hydrocarbon stream with the selective extraction solvent in a tall, usually vertical multi trayed or packed treater tower. Massive quantities of hydrocarbon feed are intimately mixed with equally massive volumes of extraction solvent to produce multi phase emulsions which, at the extreme ends of the treater tower separate by gravity into an aromatics lean raffinate phase and an aromatics rich extract phase which also contains the bulk of the selective solvent.
Because the separation of the multi-component emulsion in the treater tower is effected by gravity, it is essential that the density differences between the hydrocarbon feed and the extraction solvent be sufficiently great so that the separation can be accomplished in a reasonable length of time. Because of this restriction and also because of the tremendous volumes of material handled, it would be desirable if aromatics could be removed from hydrocarbon streams by a more energy efficient and less complicated process.
Other techniques have been put forward to separate hydrocarbon streams into their components.
The use of selective membranes has been suggested. U.S. Pat. Nos. 2,947,687 and 3,043,891 disclose the separation of hydrocarbon mixtures by passing across the face of a non-porous membrane through which at least one component of the hydrocarbon mixture will permeate.
U.S. Pat. No. 3,043,891 teaches a process for increasing the permeation rate of saturated hydrocarbons through non-porous membranes which are capable of separating hydrocarbons according to type, and/or molecular configuration, and/or boiling point or molecular weight. The patent teaches that the permeation process is increased by contacting the membrane during the permeation process with an added hydrocarbon solvent for the membrane. This solvent may contact the membrane on the feed side, the permeate side or on both sides. Representative of such permeation accelerating solvents include aromatics and unsaturated hydrocarbons such as olefins or diolefins. The solvent is described as being a solvent for the membrane, i.e., swells the membrane.
The membranes employed are described as non-porous and include natural or synthetic rubber, gum rubber, chloroprene, neoprene, vinyl polymers such as styrene polymers, polyisobutylene, certain cellulose ethers.
The patent indicates that saturated molecules will permeate through the membrane in the following sequence of increasing selectivity: open chain highly branched hydrocarbons, &lt; open chain with lesser degree of branching; &lt; closed chain (e.g. cycloparaffins) and alkyl cycloparaffins, &lt; straight chain or normal paraffins. Use of the membrane solvent will substantially increase the permeation without substantially altering the selectivity.
U.S. Pat. No. 2,947,687 teaches the separation of hydrocarbons by type through a non-porous membrane using a membrane solvent to enhance the permeation rate. Membrane solvents include substituted hydrocarbons which are soluble in and have solvent power for the membrane. The hydrocarbon solvent is an organic compound containing one or more atoms of halogen, oxygen, sulfur or nitrogen. Thus, materials such as carbontetrachloride, alcohols, ketones, esters, ethers, carboxylic acids, mercaptans, sulfides (e.g. diethylsulfide etc.), nitropropane, nitrobenzene, acetonitrile, formamide, ethylene diamine, etc. may be employed in an amount ranging from 1 to 100% based on total solvent to hydrocarbon feed. The process may be operated at a pressure differential between the feed and permeate zone with the permeate being removed by vacuum. Alternately the permeate can be removed by a sweep stream such as steam, air, butane, etc.
The membrane is non-porous and includes natural or synthetic rubber, vinyl polymers, cellulose esters, cellulose ethers.
The process can use any hydrocarbon source as feed and the separation achieved is in the order: saturated hydrocarbons, &lt; unsaturated hydrocarbons, &lt; aromatics. Saturated hydrocarbons of approximately the same boiling range permeate in the order of increasing selectivity: branched chain, &lt; cyclic-chain, &lt; straight chain configuration, i.e., straight chain paraffins more readily permeate through the membrane.
In an example methyl cyclohexane is separated from an equal volume mixture of methyl cyclohexane and isooctane using 5% methyl ethyl ketone as solvent. An operating pressure differential of 400 mm Hg was maintained and the temperature was 52.degree. C. and 82.degree. C. The methyl cyclohexane preferentially permeated through the membrane.
U.S. Pat. No. 3,956,112 teaches a membrane solvent extraction process. The membrane solvent extraction system is utilized to separate two substantially immiscible liquids and extract a solute through a solvent swollen membrane from one solvent liquid phase to the extracting solvent liquid without direct contact between the liquid phases. The membrane is substantially non-porous. Table III of U.S. Pat. No. 3,956,112 compares the invention of '112 with competing processes One of these processes is described as direct extraction via porous partition. That process is practiced using two immiscible solvents. The driving force is the chemical potential depending on the partition coefficient of the solute in the two solvents. The process employs a porous membrane or partition wall. In that process solutes from one solvent are transferred to the extraction solvent via direct solvent contact.
U.S. Pat. No. 3,140,256 teaches a membrane separation process employing a membrane comprised of a cellulose derivative (e.g. cellulose ester or ether) modified by reaction with aldehydes, organic di isocyanate, organic monoisocyanate, organo-phosphorus chlorides and organo-sulfur chlorides. Hydrocarbon feeds can be separated into these components by type using the membrane, e.g. aromatics can be separated from unsaturated hydrocarbon (olefins or di olefins) and/or from paraffins, or branched chain aliphatic hydrocarbons can be separated from other aliphatic hydrocarbons which have a different number of branched chains. Aromatic hydrocarbons permeate more rapidly than do the saturated (i.e. paraffinic) hydrocarbons. In an example methyl cyclohexane permeated through the membrane more selectively than did iso octane.
U.S. Pat. No. 3,370,102 teaches the membrane separation of aromatics from saturates in a wide variety of feed mixtures including various petroleum fractions, naphthas, oils, and other hydrocarbon mixtures. Expressly recited in '102 is the separation of aromatics from kerosene. The process produces a permeate stream and a retentate stream and employs a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type i.e. aromatics, unsaturated, saturated by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons by type, i.e. aromatics and/or olefins from gasoline boiling range mixtures by the selective permeation of the aromatics through certain cellulose ester nonporous membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid. U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation.
U.S. Pat. No. 4,914,064 teaches polyurea/urethane membranes and their use for the separation of aromatics from non-aromatic hydrocarbon. The membrane is characterized by possessing a urea index of at least 20% but less than 100%, an aromatic carbon content of at least 15 mole %, a functional group density of at least about 10 per 1000 grams of polymer and a C.dbd.O/NH ratio of less than about 8.
Thin film composites can be prepared either from suspension deposition as taught in U.S. Pat. No. 4,861,628 or from solution deposition as taught in U.S. Pat. No. 4,837,054.
Polyurethane imide membranes and their use for aromatics/non-aromatics separation are the subject of U.S. Pat. No. 4,929,358.
Isocyanurate crosslinked polyurethane membranes and their use for the separation of aromatics from non-aromatics is the subject of U.S. Pat. No. 4,929,357.
U.S. Ser. No. 452,887, filed Dec. 19, 1989 in the names of Black and Schucker, now U.S. Pat. No. 4,962,271 teaches the selective separation of multi-ring aromatic hydrocarbons from distillates by perstraction. The multi-ring aromatics are characterized by having less than 75 mole % aromatic carbon content. Perstractive separation is through any selective membrane, preferably the aforesaid polyurea/urethane, polyurethane imides or polyurethane isocyanurates.
"Microporous Membrane Solvent Extraction" Prasad, R., et al, Separation Science and Technology 22(2&3) 619-640, 1987 examines the phenomenon of dispersion-free solvent extraction through immobilized aqueous-organic interface in a microporous hydrophobic membrane. Expressly investigated was the use of an organic-organic interface to extract aromatics as exemplified by toluene, from a hydrocarbon feedstock, as exemplified by a mixture of toluene in n-heptane, employing a microporous Celgard 2400 polypropylene membrane to partition the feed from the polar extraction solvent, which in this case was NMP. The toluene selectively permeated through the porous Celgard membrane into the NMP thereby reducing the amount of toluene in the feed (raffinate) while increasing the amount of toluene in the permeate phase (extract).